Anionic polymerization techniques have been used to synthesize polymers that are useful in the manufacture of tires. These methods advantageously allow for incorporation of a functional group at the ends of the polymer chains. These functional groups have had a beneficial impact on the performance of tires. For example, interaction between functional group and fillers has allowed for the production of tire treads exhibiting reduced hysteretic loss.
When preparing these polymers, certain initiators can impart a functional group to the head of the polymer (i.e. an end of the polymer where the initiator residue resides.) Terminating agents have also been employed to impart a functional group to the tail end of the anionically-polymerized polymer (i.e., an end nearest the location where the final monomer unit has been added to the polymer).
Where functional polymers are desired, it is advantageous to prepare polymeric compositions wherein a high percentage of the polymer molecules include the desired functional group. Where the functional group is obtained through a terminating reagent, the limiting factor may be the ability to maintain live (i.e., reactive) chain ends and/or the reaction efficiency between the terminating agent and the live polymer. Where the functional group derives from an initiator, the limiting factor is often the ability of the polymer to retain the head group. In other words, while the functional initiator may propagate a living polymer, the head group (i.e., the functional group located at the head of the polymer) may be cleaved from the polymer chain, especially at higher polymerization temperatures (e.g., above 100° C.), which temperatures are typical for many commercial anionic polymerization processes. For example, polydienyl polymers having a functional head group can be represented by the formula
where α is a functional group and H′ are allylic hydrogens. It is believed that the allylic hydrogens can be displaced by a polymer anion, and this displacement can lead to a reverse Michael Addition reaction that cleaves the functional group (α) from the polymer chain. The same sequence can also occur where a mer unit deriving from styrene is adjacent to the functional group (α), because the benzylic hydrogen of the mer unit can be displaced and also lead to a reaction that cleaves the functional group (α) from the polymer chain.
For example, U.S. Pat. No. 7,153,919 teaches the formation of a head-functionalized polymer by initiating the polymerization of conjugated diene (optionally together with styrene) with a lithiated thioacetal such as a dithiane. These functional polymers have proven to be technologically useful in the manufacture of tire treads since the head group exhibits desirable filler and/or polymer interaction thereby providing tire treads with advantageously low hysteretic loss. While these initiators were used to prepare polymer with high yields of functional head groups, the polymerization temperatures were maintained relatively low. Where higher polymerization temperatures are maintained, which are often desirable or necessary in commercial operations, the loss of head group functionality can occur.
Because polymeric compositions with a greater percentage of functional head groups would be advantageous, there is a desire to solve the problems of the prior art.